1. Field of the Invention
This invention relates to wide angle zoom lenses of long back focal distance and, more particularly, to zoom lenses suited to electronic photographic cameras.
2. Description of the Related Art
In recent years, along with the advancement of art of video cameras, many studies have been devoted to development of an electronic still camera. This camera has its effective picture frame made small in size as compared with the single lens reflex camera using 35 mm photographic film, roughly on the order of 11 mm in diagonal length. Also, the use of the through-the-lens type finder in the electronic still camera leads to the necessity of arranging a reflection mirror in front of the photosensitive material to direct part of the light entering through the photographic lens to the finder system likewise as in the single lens reflex camera. In addition thereto, a low pass filter, an infrared cut filter and other glass members are very often used in the electronic still camera. From these reasons, the photographic lens for use in the electronic still camera has, despite the relatively small picture frame, to have its back focal distance considerably elongated as compared with the single lens reflux camera using silver halide film.
Numerically speaking, in the electronic still camera having the effective picture frame of diagonal length L, the back focal distance is equal to 2.1L or thereabout, being far longer than in the conventional photographic lens.
In general, to facilitate elongation of the back focal distance, the lens system must be formed to the inverted telephoto type, as comprising a front lens unit of strong negative power and a rear lens unit of positive power.
However, if the refractive power of each of the lens units is strengthened in order to obtain an adequately long back focal distance, the difficulty of correcting aberrations is increased.
In connection with the zoom lens for use in the single lens reflex camera, it is to be noted that a compact form of wide angle zoom lens is disclosed in U.S. patent application Ser. No. 450,742 filed on Dec. 17, 1982 and assigned to the assignee of the present invention.